Interferometer using liquid mirrors



Aug 4, '1953 T. w. zoal-:L ET AL 2,647,434

INTERFEROMETER USING LIQUID MIRRORS Filed July 25, 1950 3 Sheets-Sheet l ma@ w A118- 4, 1953 T. w. zoBEl. ET AL 2,647,434

INTERFEROMETER USING LIQUID MIRRORS Filed July 25, 1950 3 Sheet's-Sheet 2 Aug. 4, 1953 T. w. zoBEl. ET AL INTERFEROMETER USING LIQUIDl MIRRORS 3.Sheets-Sheet 3 Filed July 25, 1950 Patented` A'ug. 4, i953 n v c 1'? Chime 01258-121957 l o eBusiness-),f.. t

The invention described herein maybemanu-:f surfacei-mustbe accurate Within .se of a wave fatured and useii` byfofr forthfefGovernnnt Y forfgovernrnental purposs1-WithoutVi-payment'ftoe'as' invention relates to interferometer-ap-A 5 buil'tl' tlr'eflight that is used; It can thus be us rof any royalty` thereon. 1'- when increased size..l interference apparatus are paratusand more particularly to? an ciptic'a'l-,are wIflwfor'instance, an interference apparatus is rangement utilizing liquid reiiecting' surfaces:to.l designed with affo'ur times larger covering linearA produce a satisfactory int'erferorneteY fappStratus-4 dinensionpth'en thearea of the field will b el6 times'lgre'ater. 'Ifv the same raccuracy of the recapable of coveringlargev areas.` 1:.'

The criterion of the optical quality of an inter- 10 flotingfsurfaces must `be obtained, `or in ,other ferometer apparatus isdetermined by the'fquality wor-'d 'if theradius (of the curvature ofthe reof theinal interference patl'iiyh-ich isproducedfv eoto plates andthe mirrorsv should remain the by the apparatus Whenall'of the us'e'dopti'cal sain ,l the numberfof interference fringes visible parts `are assembled and `functioning''together; n"'thse'-`surfaces`Would`increasegwith thesquare Since it is known'thatth'e vqualitsfof the surfaces 15 of"thelnear'enlargement.'. The displacement of of transparent plates for 'transmitting light there-Z the "final-interference fringes in an interference through can be lower tha1f`rtl'1o'se4 requir'edforjjthe picturewhichwas 1/4 fringe Width in the smallerf reflectedlight, the `most important aiidfsens'itiveI plates'wouldinow be four full fringe Widths.y Since surfaces Withinan interferometer 'apparatusare l/iifringeawidth wasthe maximum lirnit` allowed the vreiiecting surfaces. v'Forie-lxar'nple'; in a'fc'on'a` 20 infthisexample, `all the rreflecting surfaces must ventional interferencel apparatus 'suchfas' the nofwl be :improved by grinding andpolishingto four-plate system ofthe Mach-Zehnder typegthe this finally required: 1A; Wave length accuracy',V

most important surfaces yare tl'xe" respectivc'fre:-` f Whichmeans-.that accuracy of `the vlarger plates iiecting' surfaces of the semi-transparentrbeam of the? vexample'` must be .latimes higher than splitting and combining mirrorsorfplatesiandf- 25 the accuracviofuthe Smaller plates or, in other the refiecting surfaces of theffurllfmirrors for'i-"e- Wrdsg'eachilarge plate reiiecting surface must be fleeting the splitin Ydivergingcollnate'dpartial; limited/to l111%Wavelength over the increased light 'beams toward each tlf1er.-".The` rear -surf y cioverageeld', which corresponds to only a 1/256 faces ofv the semi-transparent-'plates:can vbe less Wave length "variation in the original small size accurately figured then thevfront reectingisur: 30 platefi Oficourse` such reflector plates andmirfacesbecause these rear surfacesv ar'e-usdionly rors .wouldfprobably'cost hundredsof thousands forfthe transmitted light which .passes through of'dollars'fto' makefiftheir production Was posthe plates. I sible atfallfand require a long time to complete.

Since -the requirements for light interference. 'For thisreason,.l as set forth; in another patent apparatus for more precise and accurate meas-w 35 applc'atonhU Patent 2,555,387 to Theodor urnents in large'rf sizes v'are inreasi'ngfin many WmZbelvpatented June'', 1951 theproposalof fields of research Work, the obtainamejiaccuiay deforming the reecting surfaces an interference of the reflecting surfaces must also be increased. apparatus'xin order to obtain optical. corrections In other Words,v increasing-the sizeof the'r'efiector ofrthehighest' :possible grade is a remarkable plates and mirrory surfaces WhenLinShed @by 40 progressi-in' the-art of,interferometry.` The degrinding and polishing can and` does' increase the formationof the. reflecting surfaces however,v is difculties in obtaining `equally "accurat appaon-Iyilhafpartialsolution to the building of prejratus, and the cost ofsuch'optical parts Lissin- Y cisionxinterferonreters, and combined interfere creased to an extreme amountfwhenextremely ence-'and-Schlier'en apparatushavinglarge sizes, high accuracy is required, especiallysii'n inter-5 While-.permittingfthe' use ofoptical parts of much ferometer apparatus of very` large sizesa'' 45 lowerimcost,andv `probably jwould reduce the ex` It is assumed, for instance, thatL the:displace.,-L pense Ain .obtaininghigh quality :opaque reiiector ment of the interference fringes of av Ik "Walle Dl'lesxinahelargerwSZeSfalSo this deformatOIl length in the `final interferenceopatterrl'is;the ofitheirefllclolsis CQIirOllablQQI Oblallnggood limit allowed. Thisk is not. the: highest .,le.fluill @Quly at medium cost', also proyiding means `tp accuracy of an interferenceapparatus bu souls compensata-1. er ,astigmatism effects in cases an example for ,explainingthe probl ,A Whereconcav mi rQrs-Inustbe used, Withtheir when larger size reecting, surface ar mad wie. ,lllleflQthe light beam',` as for` it is assumed thattmsnnal influencent 1li. e a shaped Schlieren' ihtrri length in the interference pattern; the 5, i Snif. A Sp'eeel Cidve bpeqe of all four of the 'optical reector Plates a d ea formed to'a"paranolionne.oi1 'to' ofvthelfour4 surfaces ofthefo yl 'te ture, "or afparabolicjeoncave system would; pidfllfce @Si S @Glied t0 @591111 the BXQQ maille,-

3 matical shape as required by adjusting or distorting the reflecting surface.

In spite of the fact that the deformating principle as disclosed in Patent 2,555,387 promises to be satisfactory for obtaining compensating effects in the two light beams of interferometersyside-by-side collimated light input beams which which never could be obtained by grinding ori I polishing alone, only the opaque reflecting miri rors of the interferometer apparatus can be .deformed since the deforming means would be in the path of the light beams, leaving the semior partially transparent reflecting surfaces of the mirrors for splitting the interferometer-beam.

and for recombining the same after it has passed through the test medium undisturbed and therefore any inaccuracies and` curvatures in their re-*" fleeting surfaces cannot be compensated for by deformation of these surfaces. The weight of these reflector plates curves the'surfaces, and.

therefore the accuracy of the most perfectly ground plane surfaces depends upon the position of these plates when in use in the interferometer apparatus and can only be ground to approach an optically flat surface.

Since it is always desirable -to obtain the highest possible degree of fiatness at the lowest possible cost, any curvature of the reflecting surfaces can be chosen preferably in such a way that the positions of the surfaces cause the surfaces to be flatter when in theirv working or operative position. However, the possibility of obtaining an optically flat surface for the partially transparent reflector plate, and the accurate determination of this curvature, is still a real problem since an absolute flatness of the surfaces cannot be obtained, but only an approximation thereof and the deformation means must be used additionally to correct errors in the reflecting surfaces of the opaque reflector plates.

In the present invention the reflecting surfaces of the semi-transparent mirrors of an interometer apparatus and in some instances the opaque mirrors, are replaced by liquids having opaque or partially transparent liquid reflecting surfaces. These liquids produce an absolutely flat surface which is horizontal, with all of the surfaces inherently parallel, also the size, shape, and extent of the liquid reflecting surfaces are not materially limited. The apparatus is extremely valuable when usedwith elongated testing mediums such as in a portion of an elongated wind tunnel since the length of the transparent receptacle and its semi-transparent liquid reflecting surface is not limited.

In this connection an improved arrangement of illuminating elements is provided illuminatrections ing the elongated wind tunnel test section comprising a plurality of concentrated light sources disposed in a row at one side of the test medium with a corresponding row of parabolic reflectors of rectangular outline disposed in side-by-side juxtaposed relation for producing a relative wide beam of collimated light which is projected through the test medium. At the other side of the test medium a plurality of elongated sideby-side rectangular parabolic `mirrors are disposed in a row with their focal axes disposed parallel to the collimated beam through the test section, but offset to one side thereof. For instance, one row or group of these elongated parabolic reflectors may be disposed at the light exit side of the test medium, one of the elongated reectors receiving the collimated beams from three of the square parabolic reflectors at the opposite side of the test medium. The use 'of this imleave the rectangular concave reflecting surface can be inclined downwardly onto the first beam splitting. liquid reflecting surface to form a wide collimated light beam while the other group of elongated rectangular parabolic reflecting surt faces can also be disposed in a row to each receivea portion of the recombined beams, for producing the light wave fringes and Schlieren images.'v

-Referring to the drawings, in which like reference characters refer to like parts in the several figures;

.1.Fig. V1 is a vertical `sectional View diagrammatically,` disclosing a four-plate interferometer apparatus of the Mach-Zehnder general type having a secondary beam splitting plate for obtaining the Schlieren image. The opaque mirrors in this figureare disclosed as glass reflecting plates having adjusting means for deforming the plates in order to obtain final optical coryFig.y 2 is a horizontal sectional view taken through the apparatus in the plane indicated by line 2-2 in Fig. 1 and disclosing one of the transparent bottom receptacles for receiving a transparent liquid therein having a semi-transparent liquid reflecting surface;

Fig. 3 is'a longitudinal sectional view taken on the plane indicated by line 3 3 in Fig. l, disclosing a wind tunnel section having a test object therein surrounded by a testing medium; Fig. 4 is a vertical sectional View diagrammatically disclosing a four plate interferometer system similar to Fig. -1 in which the lower opaque reflecting plate or means is formed by a liquid havinggan upper opaque'reflecting surface;

Fig. 5y is a similar vertical sectional view diagrammatically disclosing a four plate and Schlieren interferometer system in which all of the reecting surfaces are liquid reflecting surfaces;

l Fig. 6 is a fragmentary horizontal sectional view of anV improved` illuminating system which is especially adaptable for use in connection with liquid reflecting surface interferometer apparatus; v

Fig-.- `'Tis the end view of the structure shown in Fig.A 6;

. Fig. 8 is a vertical sectional view taken about in ya vertical plane through the center of one of the lens elements 8l in Fig. 6.

Fig. 9 is a fragmentary horizontal sectional view of the light exit portion for receiving collimated beams from the reflectors shown in Fig. 6 after thepassage of the wide test beam through the testmedium; and

Fig. 10..is a vertical sectional view taken approximately on the plane indicated by line Ill-l Il in Fig. 9.A

Referring more particularly to Fig. l of the drawings the reference numeral i indicates diagrammatically'a beam of collimated light originating from any suitable light source such as an optical slit having a concentrated light in the rear thereof vinV which collimating lens means is provided for directing an enlarged beam of parallel' lightv rays downwardly in a predetermined inclined or oblique direction. For instance the in'clined'collimated beam I may be produced by tire 'plural iight l.parte optiearsystem illustrated in Fig. 6, in which collimatedflight"beams.are

reflected by the side-by-sid'eV rectangular-@parabolic portions of mirrors tofproduceia wide oblique beam of c'olliinatedlight,y indicatedat' I inFig. 1. Located in the path ofthe inclinedrcolvlimated beam `I is a liquid'receptacle' Zihaving a vtransparent bottom 3', of I suitable' -riiaterialy such as a glass plate. The liquid receptacle is disposed horizontally vand containsia transparent liquid Il having a partiallytransparent liquid: refleeting surface 5- for splitting thecollim-atedi beam I into two dii/ergingcollimated'partialbeamsl Ila 'and Ib disposed in the 'same' fverti'c'zalplane;'-i

' Opaque reflector plates 6V and] are disposedrespectively in the paths of they divergiiig beams Ia and Ib for reflecting-the samertowardseach other as indicated -at YIc andI 'Id. Theopaque reflectors in this figure comprisef'optioallyflat glass opaque mirrors mounted 'in supporting ring reflector I2 is interposed across 'the converging beams Ic and Id and comprises a liquid receptacle or container having a transparent plate supporting bottom I3 disposedvhorizontally and contains a transparent liquid I4 having a partially transparent rliquid reflecting surface I5.

kThe partially transparent reflecting surface I5 reflects a portion of the partial light beam` I.c in combined relation with the portion of' the converging light beam ld which passesthrough the liquid and the partially transparent liquidfrefleeting surface I5.

A test chamber or wind tunnel section I6 is "interposed in the test beam Id and Ais adapted lto contain a testing medium'and'theobject to 'be tested, such as indicated at II'in Fig.' 3. Hori- :zontally disposed between the spaced parallel 'transparent side walls Ill-I8 of the test 'cham- '.ber I6 and the liquid receptacle I2 is a receptacle I9 having a transparentA bottom' 20; and

containing a transparent liquid 2l having a partial transparent liquid refiecting surfacef22-for reflecting a portion jof the. beam'iIdffrom the reflecting surface 22 to vforma Sc'lfilierenbeam` indicated at ie.

An elongated horizontal receptacle "-'f contains a liquid 2li having an opaque liquid reflecting surface 25 disposed in the pathofvv lthe Schlieren beam` Ie for reflecting the same in" spaced parallel relation to the recombined portionsof the converging partial lightbeams" Ic` and Id and 'is indicated as the recombined interference beam 26. The Schlieren beam'is indicated at"2I,' suitfable image screen means 28 being provided'inguls the path of the interference beam lZ'Ii'foi" receiving v the interferencer fringes thereon.H A similar' image `screen 29 is disposed in the' Schlieren beam"'.21, conventional lens and Schlieren knife''edge' means being arranged in the beam 21 to' 'produce'thef Schlieren imageon thescre'en 29;" Afcornp'ensator is disposed in the path of thereflected'partial 1 'beam Ia at 32 and comprises' a 'receptacl33having va transparent vplate 'bott'om3II', la suitable ldepth..oftheY liquidf35 and theselection ofyits refraction index providing; means: for compensa-.- tionfor the', passage ofthe dividedfflig-htI beam; I1)l through'the transparent sidewalls I;8 and I-.yof the test chamberand thel testing mediumy con-l tained therein. Inv they apparatus illustrateddn Fig. 1 itV should be observed that. the most impportant liquid reflecting surfaces. arer the-reflect.- ing surfaces 5 and I5 of the beam splittingl reflecto'rs andv these: surfaces Aare formed by liquids and arev therefore always' flat and level. The Schlieren beam splitting -plate I9 is also formed by a reflecting. liquid sor that*l these liquid reflecting surfaces are always parallel toieach other'andl lie inherently fiatiY The. opaquereflector 8 is adjustablearound its pivote. 11o-bring thetwo partial. beams in. desired phase relation. The adjustingmeans I0 for .the opaque reflector plate 6, and the adjustingrneansllil for theopaque lmirror "I providey means for making a correction forany curvature in these plates dueto sag or other reasons. Since the lower surfaces. .of the trans-parent containers. are utilized for passing light rather than reflectingthe light, thezdegfree "of "flatnessi of these surfaces does not have .to

be as `great as the ilatness required :foin the opaque and semi-transparent reflectingsurfaces.

In Figl the lower opaquereflecting surface?, as shown in Fig. 1, is replaced by a liquid having an opaque liquidreflecting surface. Since the other reflecting-surfaces areA similar to.those shown in Fig. 1 the same reference numerals are used, with` the exception fthat they are primed. A `concentrated light fsource is indicated'at 36, the Vlight passing through the light aperture 3'f'I and lens means, produces a collimated light beam I. A liquid receptacle 2 is disposed` in the path of the collimated beam I and-is provided with a transparent plate bottom 3' disposed-horizontally. The receptacle -2 contains a transparent liquid 4 having a partial transparent upper-reflecting surface 5 for splittingthe collimated beam I into two diverging collimatedpartial light beams indicated` at Iaandlb extending in the same vertical plane, the lower beam I bf travers- "ing the testing mediumvin the testing chamber container 33 -having a compensating or refracting liquid 35 thereinandstrikes the opaque reilecting surface 6 of a deformable and adjustable opaque mirror which is carried ina frame Band vpivoted at 9 havingsurface contour adjustingr means I0', the adjusting means: I Il" permitting the-adjustment-of the ilatreilecting surfacein the manner indicated in connection-with the disclosure shown in Fig'. v1. y The two partial beams Ia and Iblv are reflected respectively vbythe opaque-surfaces 6"-and drinlconverging directions in the vertical plane toward each other-to form the combined vinterference light beam-"26C A liquid receptacle I2 having a transparentbottom I3A is disposed at the `point of intersection of the beams Ia and Ib contains ra transparent liquid I 4 having a partial transparent liquid reflecting surface 15' and comprises means-for recombining the two -partial'beams'to form-the interference-beam 126'.' A-fwindtunnel section ISf'is preferably interposed inthe reflected jpor- "refracung iiquid "esterne disposed meren-.th tutti@ "ld" "f thelower 4Ibeam',"a1so a nquid recep- 'tacle I9V having a transparent bottom 20 and containing a transparent liquid 2l' having partial transparent liquid reflecting surface 22 is placed-in the beam Id after it has passed the wind tunnel section for splitting the partial beam lb to form the Schlieren beam le'. A liquid container 23 similar to the container 23 in Fig. 1, is interposed in the Schlieren beam le and contains a liquid 24 having an opaque liquid reflecting surface 25 for reflecting the Schlieren beam parallel to the combined interference beam through conventional lens means 30 to converge the Schlieren beam le through the focal point onto an image screen 2S with a Schlieren knife edge 3i touching the beam at the focal point just mentioned to produce a darkened light field and the Schlieren image of the object i1 located in the test chamber it. If desired a suitable lens system M may be interposed in the interference beam 26 so as to produce the interference fringe image on the screen 28 in the image plane of the lens l i.

Referring to the diagrammatic showing in Fig. all of the reflecting surfaces utilized are liquid reflecting surfaces and therefore all of the refleeting surfaces can be large and elongated and will be inherently flat and parallel to each other. One of the containers is preferably adjustable either vertically or in the plane of one of the two converging partial light beams so as to bring the two wave lengths of the beams into phase. In this ngure of the drawing the collimated partial beam 5l) inclines downwardly from either a single light source, or a plurality of side-by-side light sources as shown in Figs. 6 to 10 and strikes the upper partial transparent liquid reflecting surface 5I of the transparent liquid 52 which is contained in a receptacle 53 having a transparent supporting bottom 54, thus splitting the collimated beam 5B to two diverging collimated partialiight beams indicated at 55 and 55 extending in the same vertical plane.

The diverging collimated partial light beams 55 and 55 each have opaque liquid reflecting means disposed in their paths. The upwardly reflected beam 55 passes through the transparent bottom 51 of a liquid container 58 having a transparent liquid 5t therein having an upper opaque refleeting medium surface 6G supported by the liquid. The lower diverging beam 56 strikes the opaque liquid reflecting surface 5i of the liquid 62 contained in a liquid receptacle 53. The opaque liquid reflecting surfaces 6U and 6| reflect the diverging partial beams 55 and `55 toward each other as indicated at 64 and 65. The

reflected test beam 65 passes through the test chamber G6 and is split to form the Schlieren beam Eil by the semi-transparent liquid reflecting surface 68 of a transparent liquid 69 carried in a container IQ having a transparent bottom 1l.

The testing medium being located in the test interposed in the path of the Schlieren beam to"4 converging Schlierenbeam 61 at the focal point,

providing the darkened light field for producing the Schlieren image.

Referring to Figs. 6 to 10, a row of parabolic reflectors of substantially rectangular outline is indicated by the reference .numerals 8|. The

contour of these parabolic reflectors each constitute a rectangular portion of a parabolic refleeting surface which is located at one side of the axis 82 of the whole parabola 83 so that a light source may be interposed at each of the focal points Sil of the respective reflectors 8l to produce a plurality of juxtaposed parallel beams of collimated light, projected by the respective side-by-side reflectors to form a wide collimated beam 8a for use as a wide beam of light for the interferometer apparatus. Each wide beam or multiple beam will be projected downwardly onto the first beam splitting liquid reflector surface 5, shown in Fig. 2 of the drawings.

A somewhat similarly formed group of partial parabolic reflectors 85 is interposed in the exit portion of the wide interference light beam, shown more particularly in Figs. 9 and 10. These parabolic reflectors, like the reflectors 8| have their axes 8l offset so as to pass outside of the confines of the elongated rectangular reflector portions and the reflectors may be disposed in the path of the interference beam, such as shown at 26 in Fig. l, or in the path of the interference beam 8, in Fig. l0, so as to reflect the Same through suitable focal points or lens means onto an image screen for observing, or recording, the light wave interference phenomena.

As shown in Fig. 9 the length of the juxtaposed rectangular parabolic reflector portions 86 is greater than the length of the parabolic reflectors Eil so that each longer reflector may be disposed in the path of a plurality of the collimated beams t5 which are reflected by the respective shorter reflectors Si. In this way a plurality of light sources may be utilized, disposed at the respective focal points 84 and thus materially increase the amount of illumination in the interference beams forming the wide beam which strikes the longer rectangular reflectors 86, thus increasing the emciency of the apparatus. In employing a plurality of liquid reflecting surfaces in conjunction with a wide beam formed by a plurality of side-by-side rectangular reflectors, each having a large aperture, it is possible to produce an interferometer apparatus for covering large, and particularly long test chambers such as elongated wind tunnel sections and obtain intense illumination. Since the reflecting surfaces are all liquids they are inherently level and parallel, thus providing a feature of self-alignment. The adjustment of one of the liquid containers to equalize the linear distance of the two split beams is a comparatively coarse adjustment since the liquid reecting surface is always level and parallel with the other reflecting surfaces. In the transparent liquids a partial coating of a very fine metallic powder or a film of suitable liquid may be used which is deposited onto the surface of the transparent liquid, the amount of this powder or reflecting medium determining the proportion of the light beams striking the liquid surface that will respectively pass through and the proportion of the beam that will be reflected. In ythe opaque liquid reflectors used the amount of the reflecting surface powder may be increased to forma perfect opaque reflector, at the upper surface,`facing upwardly, or facing downwardly, in which last event the container would have a transparentbottom and contain a transparent asap-484 :9 liquid. l @-Whenthecupper: liquidfreflectingsurface faces; upwardly: andi-is: opaque fthe `container and the li-liquid may also be opaque since the z'light beam. strikesithis latter surface` from above i and .isntullyl-reiiected. lIhis liquid'may `bei'mercury. fftheacontaineriswide enough the usable'part off/the liquid'ywill be perfectlyflat and. level. l"The lower or supportingl surfaces for the transparent bottom containers may be fairly accurately groundandpolished glass platesvhaving as little curvature, due to the weight of the plate and the liquid thereon as possible.

'.It is of course necessary,.in an interferometer device-using liquid reflecting surfaces to provide asrmandfsolid a support for-the liquid containers aspossible so that vibrationy cannot disturb the liquid reiiecting surfaces.

lIn the-liquidcompensatorit is only necessary to-'select any` suitable' liquid having Y'a desirable lightv refracting index and adjust the quantity ofthe liquid in the compensator receptaclel so as tobring the wave length of the widetestpartial beampassing through the testing medium into phase with the .otherwide partial beam not traversing the test medium. An v interference fringe image can: then be observed on the screen in'the Wide recombined main beam, Vand a Schlieren imagecan :be -observed on the image screen in the reiiected' Schlierenfbeam.

Havingrr thus Adescribed :our invention,y what We claim as new and desire to secure by .Letters Patent is:

1,1. In an optical systemfor thevin'vestigationof lightr ldensity- 'ields of f atest medium. comprising means for producing an inclined input collimated light bea-m; a Vfirst partially transparent liquid reflector having its reflecting surface `supported tol seek av level horizontally: across the input; light beam for splitting-- the vcollimate'd lightbeam into anupwardly inclined reflectedpartial pencil of collimated ylightand a" second `relatively diver-ging downwardly inclined-"partialw pencil of -`collin'lated iight passi-ng through'the'liquid` reflector; `a pair of Uopticallydiat "reflectors, `one disposed across each-of the aforesaid lightjpencils in a horizontal position'for` reiiecting, said light pencilstoward each-other, a second partially transparentlliq'uid .Ieiiector supported to seek alevel'horizontally in both of the partial v lightpenc'ils at, the point of intersectionl thereof .to recombinefthe partial light pencilsby reectingone of the partial light pencils upwardly in.` an inclined direction in Va vertical plane,.,and. passing. rthe other. partial light pencil upwardly therethrough in the: same inclined-direction; atestchamber for receiving a ,tes-ting; medium having transparent lside walls disposed one vof they aforesaidzcolli-mated par- Ltial i light pencils perpendicularl tothepencilfand 'located betvveen'thef saidrst partially. transparent. liquid reflector and :said secondfpartially .transparentliquid :reecton means in the path of theyother-partial 'light` pencil to :compensate `for,variationsfingphase -inthe two partialxlight pencils caused bythe passage .of f one partialI light pencil throughF the vtest cham-berV and :the :nonpassage of the otherI partial light pencilf through :the test chamber; oneof the ssaidy horizontal par- .:tially i transparent z liquid vreflectors beingl shift- ..able.transversely to-the planeof its reflecting 4:surface for; adjusting the: relative.Y linear distance ofi the two partial light pencils. betweenl .thefrsaid partially transparent liquid. reflectors ;l lens .means :located inlitherx path Yofi-,tige reombndzpartial 1G light pencils forproducinga: light image of the recombined partial:y light-- pencils.

=2. vApparatus as claimed in claimfl wherein the said partially transparent liquid reflectors are disposed horizontally in horizontally spaced relations'at thefopposite `corners offa verticalfparallelogram in'which the axis ofthe inclined collimated input light beam is` coincident with one side Aof the parallelograinwith one corner ofthe parallelogram disposedat the reflecting surface of the first; mentioned partially transparent liquid reflector, vthe said second partially transparent liquid reflector being disposed in horizontally spaced: relation to the' first partially transparent liquid reiiectory at the opposite corner of the parallelogram, and one of the said opaque fiat reflectors is disposed horizontally vateach of the remaining opposite corners of the parallelogram.

r3. Apparatus asclaiined in claim 2- in which all of fsaid reflectorscomprise liquid supporting receptaclesv disposedsubstantially horizontally and containing a liquid therein having a light reflectirflgsurface seeking its level horizontallywhereby all of the reiiecting surfaces are `free to-seektrue flat-horizontal planes.

4; Apparatus .as claimed in claim -3 in which at least three of the liquid receptacles each include a; transparent lower liquid reflector supporting surface to permit passage of the-inclined Vlight beamstherethrough and each of; the three recep tacles contain a transparent liquid having an upper light reflecting surface.

v5. vIn an optical system for the investigationlof light density fields of a test medium bythe wave length vinterference method; lens means for collimating light from a Alightsource into aninclined Ibundle of parallel light-rayslextendingin avertical-nplane; a lrst lliquid =receptacle having a [transparent vsubstantially horizontal bottom Itherein 4fired in the path of the parallel light rays, and adapted to receive a transparent liquid therein having a partially transparent upper reiiectingsurface disposed inthe path ofthe parallel light rays for splitting the parallel light rays linto two partial light pencils of-collimated light .diverging'from'each other in the said vertical plane, one'partial lightv pencil-passing through the-receptacle, liquidand its reflecting surface, .and the otherpartial lightpencil being reected by7 the reflecting surface ofthe reflecting liquid inthe'aforesaid vertical plane,y a pair of optical `flat reflectors-disposed in vertically spaced parallel relation to the partially transparent reflect ingsurface of the liquid, one above and the other belowyeach `reflector of saidfpair having a horizontalV reflecting surface disposed'in lthe path of kone of the diverging partial light pencils for reflecting the twopartialf light pencils toward each .other in the--said verticalv plane; at least one reiiector of said pair comprising a substantially horizontal liquid holdingreceptacle'adapted to receivea liquidtherein having an opaque upper yreflecting surface; means for recombining said partial light pencils at their point of intersection comprising-asecond horizontally disposed receptacle having a transparent bottom "therein, and

adapted to containa transparent liquid'having a partially-transparent -reiiecting surface located at the intersection ofthe partial light pencils for reflecting one of Ithe partial light pencils in the -fvertical plane and passing the'other light pencil Atherethrough in combined -collirnated relation with the other last mentioned reflectedfpartial .light penciLaitesting chamber 'disposed in the path of one of the i partial llight pencils, hav- Hf ing spaced parallel transparent side walls and adapted to receive a test medium therebetween; a transparent receptacle disposed in the other partial light pencil having a transparent bottom therein and adapted to contain a predetermined quantity of transparent liquid therein for the passage of said other partial light pencil therethrough, to compensate for the dierence in phase between the two combined partial light pencils incident to the passage of one of the partial light pencils through the testing chamber and the passage of the other partial ligm pencil exteriorly of the walls of the test chamber; an additional liquid container having a transparent bottom, located horizontally in the path of the partial light pencil passing through the testing chamber, and located between said testing chamber and the recombining reflecting surface and adapted to contain, a transparent liquid having a partially transparent liquid reflecting surface for reflecting a portion of the partial light pencil passing through the testing chamber, out of that light pencil in the vertical plane, and another liquid container disposed in the last reflected path of the last mentioned partial light pencil adapted to contain a quantity of liquid having an opaque light reflecting surface for reflecting the last mentioned reflected partial light pencil v separately in an adjacent path parallel to the recombined partial light pencils to form a Schlieren interference light beam.

6. Apparatus as claimed in claim 5 including lens means disposed in the path of the recombined partial light pencil, and in the adjacent reflected partial light pencil for converging the recombined light pencils and the parallel adjacent light pencil through focal points to form a pair of parallel juxtaposed light images at the image plane of the lens means, whereby when la Schlieren knife edge is positioned to engage the said adjacent partial light pencil at the focal point of the lens means therein produces a Schlieren image and an interference image in said image plane.

'7. In an optical system for the investigation of light density field of a test medium by the wave length interference method, means for producing an inclined collimated light beam; a rst partially transparent reflector comprising a transparent liquid having a partially transparent upper reflecting surface adapted to seek its own level and disposed in the path of the inclined collimated light beam for splitting said collimated light beam into a pair of diverging partial light pencils extending in the same vertical plane; a xed supporting receptacle for said liquid having a transparent bottom; means for recombining the light pencils comprising a pair of horizontal opaque optically nat reflectors, one disposed in the path of each of the diverging partial light pencils for reflecting each of the diverging partial light pencils obliquely toward each other in the aforesaid vertical plane, a second transparent liquid having a partially transparent upper rellecting surface adapted to seek its level in a horizontal plane passing through the point of intersection of the partial light pencils after reflection thereof by the pair of horizontal opaque optically fiat reflectors, a second supporting receptacle having a transparent bottom for supporting the last mentioned transparent liquid; at least one of the aforesaid pair of horizontal opaque optically flat reflectors comprising a liquid having an opaque reflecting surface disposed in the path of one of the partial light pencils and a liquid receptacle for retaining that liquid therein, said partially transparent reflecting surface of said second transparent liquid passing a portion of one of the partial light pencils obliquely therethrough in said vertical plane and reflecting a portion of the other partial light pencil obliquely therefrom in said vertical plane in recombined relation; a testing chamber having transparent side walls disposed in the path of one of the partial light pencils intermediate the said second partially transparent liquid reflecting surface and the reflecting surface of the horizontal opaque optical flat reflector of said pair which is located in that partial light pencil and adapted to receive a testing medium; wave length phase compensating means disposed in the path of the other partial light pencil for the oblique passage of that partial light pencil therethrough, located between the said first and second transparent liquids, said Wave length phase compensating means comprising a light refracting transparent liqiud having a predetermined depth for the oblique passage of that partial light pencil therethrough to compensate for rela,- tive wave length phase difference between the two partial light pencils caused by the passage of the other partial light pencil through the walls of the test chamber; and a liquid receptacle having a transparent bottom for supporting the refracting liquid in the path of the partial light pencil which does not traverse the testing chamber and testing medium.

8. Apparatus as claimed in claim '7, including an additional transparent liquid having a partially transparent reflecting surface adapted to seek its level horizontally in the path of the partial light pencil passing through the testing chamber, and located between the testing chamber and the partially transparent reflecting surface of the lsecond transparent liquid, for reflecting a portion of that partial light pencil obliquely out of that lightv pencil, in the said vertical plane, transparent liquid container means for supporting said additional transparent liquid in last mentioned partial light pencil; and another reflecting liquid having an opaque reflecting surface adapted to seek its level horizontally below the partially transparent reflecting surface of said additional transparent liquid, for reflecting the portion of the partial light pencil which is reflected by the partially transparent reilecting surface of the additional transparent liquid, in an adjacent parallel relation to the partial light pencil passing through the last mentioned partially transparent reflecting surface.

9. In an optical system for the investigation and analysis of the light density fields of a test medium by the light wave interference method; lens means for collimating light from a light source into an inclined beam of parallel light rays; a first liquid receptacle having a transparent bottom disposed substantially horizontal in the path of said inclined light beam; a transparent liquid in said first receptacle having a partially transparent upper light reflecting surface located in the path of said inclined light beam adapted to seek its level horizontally for splitting said inclined collimated light beam into two inclined collimated partial light beams, one partial light beam being rellected by the light reflecting surface of the liquid in a diverging direction in a vertical plane relative to the other partial light beam, the said ether partial light beam passing through the light reflecting surface, the liquid and the bottom of the receptacle in the same vertical plane; a second receptacle located in a horizontal plane above said first receptacle, a transparent bottom in said second receptacle disposed in the path of the aforesaid one of the diverging light beams, a transparent liquid in said second receptacle, an opaque light reflecting medium supported on the upper surface of the last mentioned liquid for reflecting that light beam downwardly through the liquid in an inclined direction in said vertical plane; a third receptacle located in a horizontal plane below said rst receptacle; a liquid therein having an upper light reflecting surface adapted to seek its level horizontally, said third receptacle being disposed to position the reflecting surface of the liquid therein in the path of the said other partial light beam to reflect the same in said vertical plane toward the other reflected partial light beam to converge the two partial beams; a partially transparent reilector located horizontally in the path of both of the partial light pencils comprising a horizontally disposed liquid receptacle having a transparent bottom and a transparent liquid contained in the last mentioned receptacle having an upper partially transparent reflecting surface adapted to seek its level in the path of both of the partial light beams at the point of intersection thereof for recombining the two partial light beams to form an interference light beam; lens means disposed in the path of the recombined means for projecting said recombined partial light beams through a focal point to an image plane; a testing chamber having transparent opposite side walls disposed in the path of one of the collimated partial light beams for the passage of that partial light beam therethrough, adapted to contain a testing medium; and compensating means in the path of the other partial light beam for adjusting the relative optical length of the two partial light beams.

10. In an optical system for the investigation of light density elds of a test medium by the light wave interference method; lens means for collimating light from a light source into an inclined collimated light beam; a first transparent liquid receptacle horizontally interposed in said inclined collimated light beam; a transparent liquid therein adapted to seek its level, a partially transparent reflecting medium supported on the surface of said liquid having a reflecting surface for splitting said collimated light into two diverging collimated partial light beams, one beam passing through the liquid, reflecting medium, and the receptacle, and the other beam being reflected by the said reflecting surface in the same vertical plane; a second transparent receptacle disposed in the path of the reflected partial light beam; Ia transparent liquid therein; an opaque reflecting medium having a light reflecting surface supported on the surface of the last mentioned liquid for reflecting said reflected light beam away from that reflecting surface in the same vertical plane; -a third liquid receptacle disposed horizontally in the path of the partial light beam which passes through the rst liquid receptacle and the transparent liquid and partially transparent reflecting medium contained thereon; a liquid contained in the third liquid receptacle adapted to seek its level horizontally; an opaque reecting medium supported on the surface of the last mentioned liquid having a light ,reflecting surface, said second and third receptacles being disposed in vertically spaced relation to the firstl receptacle, and in vertically spaced relation to each other in the same vertical plane to dispose the reflecting surfaces of the liquid supported opaque reilecting mediums to reflect the collimated partial light beams toward and across each other in intersecting relation, vthe light reflecting surfaces of said opaque light reflecting mediums being positioned relative the refleeting surface of the partially transparent refleeting medium in the first receptacle being such that the linear distances of the said two partial light beams along their axes to their point of intersection are substantially equal; a fourth liquid receptacle disposed horizontally in the path of both partial light beams, a transparent bottom therefor; a transparent liquid therein adapted to seek its level horizontally a partially transparent reflecting medium supported on the last mentioned transparent liquid, positioned to locate the reflecting surface of the liquid supported partially transparent refleeting medium substantially at the point of intersection of the two partial light beams to recombine the partial light beams to form an interference beam; said fourth receptacle being adjustable in the direction of one of the reflected partial light beams for adjusting the relative optical distance of the two partial light beams from the partially transparent reflecting surface on the liquid in said first receptacle to the reilecting surface on the liquid in said fourth receptacle; a transparent testing chamber disposed in the path of one of the partial light pencils for receiving a testing medium; compensating means in the other partial light pencil to compensate for change in optical distance of the testing chamber traversing partial beam due to its passage through the testing chamber, comprising a liquid receptacle, a transparent bottom therein for the passage of the last mentioned partial light beam therethrough; a transparent liquid in said last receptacle for passage of the last mentioned partial light beam therethrough, whereby the depth of said last mentioned liquid, including the thickness of the transparent bottom of the last mentioned receptacle, compensates for the passage of the testing chamber traversing partial light beams through the transparent `walls of the test chamber,` and lens means disposed in the recombined partial light beams, for producing a visual picture of the light wave interference between the partial light beams when a test medium is interposed in the testing chamber.

THEODOR W. ZOBEL. FERDINAND lVEIRUSl.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,044,502 Crehore et al Nov. 19, 1912 1,598,106 Schoonmaker Aug. 31, 1926 1,939,088 Styer Dec. 12, 1933 2,015,730 Rosin et al. Oct. 1, 1935 2,256,855 Zobel Sept. 23, 1941 2,460,836 Lovins Feb. 8, 1949 2,541,437 Prescott Feb. 13, 1951 2,555,387 Zobel June 5, 1951 FOREIGN PATENTS Number Country Date 389,529 Germany Feb. 4, 1924 888,644 France Sept. 13, 1943 

